Composite seal for turbomachinery

ABSTRACT

A gas path seal suitable for use with a turbine engine or compressor is provided. A shroud wearable or abradable by the abrasion of the rotor blades of the turbine or compressor shrouds the rotor blades. A compliant backing surrounds the shroud. The backing is a yieldingly deformable porous material covered with a thin ductile layer. A mounting fixture surrounds the backing.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the U.S.Government and may be manufactured and used by or for the Government forgovernmental purposes without the payment of any royalties thereon ortherefor.

RELATED APPLICATION

This application is a division of copending application Ser. No. 931,090filed Aug. 8, 1978, now U.S. Pat. No. 4,207,024, and which, in turn, isa division of application Ser. No. 801,290 filed May 27, 1977 and nowU.S. Pat. No. 4,135,851.

BACKGROUND OF THE INVENTION

In the design of turbines or compressors or the like, especially thoseof high speed, it is understood that close tolerance between the tips ofthe blades and the surrounding shroud or housing which seals one side ofthe blades from the other is desirable. Such a seal reduces the returnflow of fluid from the high pressure to the low pressure side. Thecloser the shroud surrounds the tips of the blades, the more efficientis the turbine or compressor. Aerodynamic losses are also reduced bycloser fitting of the blade tips to the shroud.

Nevertheless the clearance dimensions are dynamic. They increase anddecrease with temperature and with mechanical and aerodynamic forces andmay increase or decrease faster than the rotor. The clearance may alsodecrease, for example in one direction, under so-called shock loadingwhen an airplane makes a sudden maneuver that subjects the parts tounusual acceleration forces, causing a differential expansion ormovement of the parts.

The prior attack on such problems has been to design the shroud to fitclosely, say within 20 to 30 mils. (i.e. about 5 to 7 mm.) about thetips of the blades at ambient temperature. Moreover, the shroud aboutthe blade has been designed to be wearable or abradable relative to theblade tips. Then if there is a thermal transient or shock loading thatcauses a blade tip to strike the shroud, the blade material flakes offor abrades the shroud material, which may be a sprayed coating orsintered material of low density. Thus, the shroud material is abradable(or wearable) with respect to the blade material. However, such wear isnot readily achieved in practice. By the arrangement of the presentinvention the blade tip may be designed to rotate in close proximity tothe shroud without fear of cataclysmic destruction of either blade orshroud, should contact between the two occur due to thermal transient orshock loading.

SUMMARY OF THE INVENTION

According to the present invention, there is provided in combination fora turbine rotor or the like a stator shroud of material wearablerelative to the blades, a compliant backing of a yieldingly deformableporous material with a thin ductile layer, and a rigid mountingsurrounding the compliant backing. The compliance of the backing tendsto cause the shroud when tipped or touched by the rotating blade becauseof some transient condition to yield somewhat. Then, even though abradedor worn, the compliance of the backing provides a restorative forcetending to return the shroud at its worn portion toward its originaldimension. In the preferred embodiment wherein the compliant backing hasa ductile top coating, the normal forces are limited and the blade wearis reduced. Hence any gap is less than in prior instance of such wear,and the useful life of the shroud is increased.

BRIEF DESCRIPTION OF THE DRAWING

The objects, advantages and novel features of the invention will be morefully apparent from the following detailed description when read inconnection with the accompanying drawing which is a schematic view, inlongitudinal cross section, of a preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the FIGURE, a rotor blade 10 of a turbine compressor, forexample, rotates about an axis 12 tending to force the fluid in which itoperates along this axis. A shroud surrounds the blade 10 and issubstantially concentric with the axis 12. The shroud may be of suitablematerial abradable relative to the material of the blade 10.

A compliant backing surrounds the shroud. The abradable shroud is bondedto the concave, inner face of the backing. The backing is inserted andbound to a housing 22.

Previous abradable seals, even though various materials for the shroudhave been proposed, such as sintered metals, metal honeycombs, andporous ceramics, have not provided a desirable flexibility. Afterabrasion due to a transient condition such as a thermal transient orshock loading, the gap or wear produced by the rub or contact isgenerally larger than the interference depth because of tearing out,galling, and spalling.

The drawing illustrates the preferred embodiment in which the blade 10is shown at an instant when forces have caused the tip of the blade tocontact the shroud. The compliant backing in this case is slightlycompressed, the compression as illustrated in the drawing in exaggeratedfashion.

The material of the shroud 34 may be a relatively thin layer 36 ofdeformable metallic material. The backing 38 is a compliant material,such as low density sintered metal, or gas blown plastic. The metal ormetal alloy layer 36 may be a dense plasma sprayed soft metal, such asaluminum, or a bonded metal sheet or foil.

When the blades, as a result of forces such as described hereinbefore,rub against the shroud, the thin metallic layer 36 is deformed into thecompliant substrate material 38. In effect because of the compliantsubstrate material 38 the magnitude of a force normal to the surface ofthe shroud is limited. This normal force is measured in the radiallyinward direction. A larger displacement simply causes a greaterdeformation without greatly increasing the normal force. Therefore, ineffect, the compliant substrate material limits the magnitude of thenormal force which can be developed between the blade tip and theshroud. The more porous and yieldingly deformable the substrate 38 andthe thinner the shroud layer 36, the smaller will be the normal force.Because the normal force is low when the blades rub against the shroud,blade wear is reduced.

Currently used shroud materials such as sintered metal or felt metalshave disadvantages of low erosion resistance, aerodynamic roughness andporosity which permit back leakage. An arrangement as described inconnection with the drawing with the shroud layer as suggested hereinprovides an aerodynamically smooth surface and high erosion resistanceand is substantially less permeable from a leakage standpoint.Furthermore, any wear debris produced in the rub is less than thatproduced in other shroud arrangements. Wear debris tends to plug upturbine cooling holes and to plate out or stick on compressor bladescausing aerodynamic losses.

Various modifications may be made. For example, metallic sheets (notillustrated) may be distorted by wires aligned parallel to axis 12 toform corrugations for the compliant backing, or these corrugations mightextend at angles of less than 90° to the axial direction. The compliantbacking 38 may use fibermetal, and instead of the thin metallic layer,material such as high temperature resistant ceramic may be used.

What is claimed is:
 1. A gas path seal for a turbine or the like havinga plurality of blades mounted for rotation about an axis comprisingastator shroud having an aerodynamic smooth surface on a thin layer of adeformable metallic material that is wearable relative to said bladesand closely spaced to the blade tips whereby said shroud is deformedwhen said blades rub against said surface, a yieldingly deformableporous backing surrounding said layer to receive said deformed shroudthereby reducing said blade wear by limiting the magnitude of normalforces measured radially inward which are developed between the bladetips and the shroud, and a rigid mounting housing surrounding saidbacking and being secured to the outermost surface of said backing. 2.The combination as claimed in claim 1, said shroud being substantiallycylindrical.
 3. The combination as claimed in claim 2, said mountinghousing being also cylindrical.
 4. The combination as claimed in claim1, said yieldingly deformable porous backing being metal.
 5. Thecombination as claimed in claim 4, said metallic backing materialcomprising a thermal sprayed layer.
 6. The combination as claimed inclaim 5, said metallic backing material comprising porous sinteredmetal.
 7. The combination as claimed in claim 1, said yieldinglydeformable backing material comprising a gas blown plastic material. 8.The combination as claimed in claim 1, said stator shroud materialcomprising a dense plasma sprayed aluminum layer.
 9. The combination asclaimed in claim 1, said shroud material comprising sheet metal.
 10. Thecombination as claimed in claim 9, said shroud material comprisingaluminum foil.